The chemical composition of organic polymers make them easily combustible. A wide variety of flame retardant additives are blend into plastics to achieve stringent standards demanded by processors and even by legislation.
Recently, despite the fact that halogen free flame retardant agents/additives (FR) represent, generally speaking, more expensive solutions compared to traditional brominated FR, some preference has been given to halogen free flame retardant additives because of environmental as well as for safety reasons. As a consequence, halogen free flame retardant (FR) agents are of increasing interest in the thermoplastic polymers market. Basic requirements for these products are good processing characteristics in compounding and moulding conditions, good mechanical and electrical properties in the solid state, no blooming or discoloration, good flame retardant properties in both reinforced and unreinforced polymers.
Particularly, flame retardant agents commonly used in Polyamides must satisfy stringent requirements concerning high processing temperatures, the absence of residual acidity that could cause polymer hydrolysis and the avoidance of blooming effects. These various requirements have limited the use of most of the known halogen free additives, apart from Red Phosphorus and Organic phosphinates.
Red phosphorus is the most effective flame retardant additive, especially for glass fiber reinforced polyamide, but its inherent red colour makes the corresponding polymer product difficult to be used for very light colours applications.
Organic phosphinates are a new family of halogen free flame retardant agents, particularly efficient in polyamides, especially when used in conjunction with a melamine containing product, the resulting mixture being more effective than organic phosphinates alone. These products, with particular regard to zinc and especially aluminium diethyl phosphinate, are currently commercially available under the “Exolit OP” brand.
This family of organic phosphinates was firstly described as flame retardant agents for example in EP 699708 and EP 6568.
Different processes for the preparation of Organic phosphinates were described, for instance in: CA 2641112, U.S. Pat. No. 6,300,516, U.S. Pat. No. 5,973,194, U.S. Pat. No. 6,011,72, DE 19910232, U.S. Pat. No. 6,090,968, EP 1016623, U.S. Pat. No. 6,329,544, U.S. Pat. No. 6,355,832, U.S. Pat. No. 6,359,171, U.S. Pat. No. 6,278,012, US 2003073865, US 2002079480, US 2006074157, US 2005137418.
Despite Organic phosphinates are satisfactory in terms of overall performances in polyamides, however, there is still a need for simpler molecules that could be produced in an easier and less expensive way.
Hypophosphorus acid metal salts, also called inorganic Phosphinates or Hypophosphites (phosphorus valence state=+1) have been reported as effective halogen free flame retardant additives for polymers.
Hypophosphites have the following chemical formula:Me(H2PO2)n where:
“n” is an integer number ranging from 1 to 4 in dependence of the valence of the metal Me. The metal is any atom belonging to the groups I, II, III and IV of the periodic table of the elements.
Sodium and calcium Hypophosphites are currently widely commercially available and they are normally produced by reacting the corresponding metal hydroxide on yellow phosphorus, for instance according to the following reaction scheme:P4+2Ca(OH)2+H2O→Ca(H2PO2)2+CaHPO3+PH3 
Hypophosphites of metals other than Calcium and Sodium are normally produced through the hypophosphorus acid reaction on the metal hydroxide or by exchange reaction with the corresponding soluble metal salts (see for instance “Hypophosphorus Acid and its salts”, Russian Chemical Review, 44 (12), 1975).
Hypophosphites are a simpler molecules compared to organic phosphinates and are also easier to be produced. In addition, hypophosphites may be also used as starting raw material for subsequent organic phosphinates production.
Thermoplastic polyamides moulding materials containing Hypophosphites as flame retardant agents, were described in the art, see for instance WO 09/010812 and WO 2005/075566,
According to WO 2005/075566, a polyamide is made flame retardant by the addition of at least Aluminium Hypophosphite in the range of 1% to 30% by weight.
In all the conditions described in the previous art however, Hypophosphites result in lower flammability performances compared to organic phosphinates when tested on Polyamide 6. Particularly, no V0 rating according to UL-94 is achieved when the thickness of the speciment is lower than about 1 mm on Polyamide 6. This lack of flame retardant performances has been a strong barrier to the full development of flame retardant compounds based on Hypophosphites when used in combination with polyamides. There is therefore a strong need for highly performing flame retardant products, based on polyamide polymers and Hypophosphites.